Method of controlling drying stresses by restricting shrinkage of ceramic coating

ABSTRACT

A method of controlling drying stresses through controlled shrinkage of a ceramic coating coated onto a substrate. The method first includes applying a non-cured ceramic coating onto the substrate and permitting the coating to dry until a mechanically stable outer surface layer of the coating is formed. Once this layer is formed, a drying control is applied in a quantity sufficient to penetrate into the surface layer, with the result of such application being the inhibition of formation of a dry outer skin as continued drying is permitted to occur. Once such drying is complete, the outer surface layer is subjected to an elevated temperature for curing and for evaporating therefrom the drying control agent to thereby produce a cured-ceramic coated substrate whose coating is void of large cracks and whose durability is beneficially opportune.

STATEMENT OF GOVERNMENT RIGHTS

[0001] This invention was made with Government support under contractF33657-81-C-0067 awarded by the United States Air Force. The Governmenthas certain rights in this invention.

CROSS-REFERENCE TO RELATED APPLICATIONS (Not Applicable) BACKGROUND OFTHE INVENTION

[0002] This invention relates in general to the application of ceramiccoatings onto substrates, and in particular to methodology forcontrolling drying stress during shrinkage of a ceramic coating soapplied through application of a drying control agent onto the outersurface of the coating after the coating is placed and minimally dried,and thereafter drying and heat-curing the coating.

[0003] Ceramic coatings are regularly applied to various substrates in avariety of product areas. One such product area is in aircraftconstruction, and includes such coatings on jet engine and engineexhaust components for thermal protection and energy absorption. Oneprior art method of coating placement involves the application ofwater-diluted, cement-based ceramic slurries by spraying or brushing theslurry to accomplish placement thereof, followed by ambient environmentevaporation of the water of solution and final elevated-temperaturecuring to remove the water of hydration. The cements of the slurriestypically are alkali metal silicates and bind together included ceramicoxides and energy absorbing fillers chosen for an intended application.The cement, or another compatible cement placed as a primer on thesubstrate, provides adhesion to the substrate.

[0004] Drying and curing a typical coating as described above results involumetric reduction, i.e., shrinkage of the cement portion material andsubsequent crack formation thereof. Cracking can also result fromdifferences in thermal expansion of the substrate and the coating aswell as from distortion or deformation of the substrate that mayaccompany heating and cooling thereof during a cure cycle. Once a crackpenetrates the thickness of the coating, the crack then has a tendencyto “curl” or branch into cracks parallel to the coating-substrateinterface. Such parallel cracks can result in an immediate loss ofcoating during cool-down from a cure temperature as well as during usewhere thermal cycling or high vibration occurs.

[0005] Generally, the characteristics of cracks in cement-based coatingsrelate to the total amount of shrinkage that occurs during the loss ofthe water of hydration, and additionally relies upon the coated surfacearea, the coating thickness, and the coating strength and condition atthe time when cracking occurs. The amount of shrinkage is related to thewater of solution initially in the slurry, the water of hydration thatis removed during curing, and the strength of the material that isresisting shrinkage when the water is removed. Material that is weakwhen cracking begins may form many small cracks, while a strong materialmay form one or more large cracks. Because drying typically occurs fromthe surface into the thickness of the coating, cracks tend to initiateat the surface during drying and curing and propagate through thethickness as the water diffuses to the surface for exit. Both largecracks in the coating and loss of coating are usual causes for rejectionof coated or would-be coated products. Thus, desirable coatings haveeither no cracks or very narrow cracks, with the latter many timesadvantageous depending upon required local flexure of the coating inaccord with characteristics of the coated substrate.

[0006] In view of present-day inefficiencies of typical ceramic coatedsubstrates, it is evident that a need is present for a method of coatingsuch substrates which produces a product with an intact coating.Accordingly, a primary object of the present invention is to providemethodology for controlling drying stresses during shrinkage of aceramic coating on a substrate.

[0007] Another object of the present invention is to provide coatingmethodology wherein a moisture preservation agent is applied onto theouter surface layer of the applied coating prior to complete drying andcuring thereof.

[0008] These and other objects of the present invention will becomeapparent throughout the description thereof which now follows.

BRIEF SUMMARY OF THE INVENTION

[0009] The present invention is a method of controlling shrinkage andsubsequent drying stresses of a ceramic coating coated onto a substrate.The method first includes applying a non-cured ceramic coating onto thesubstrate and permitting the coating to dry until a mechanically stableouter surface layer of the coating is formed. Once this stable outersurface layer is formed, a moisture control agent is generally uniformlyapplied onto the layer of the coating in a quantity sufficient topenetrate into the surface layer, with the result of such applicationbeing the inhibition of formation of a highly stressed dry outer skin ascontinued drying is permitted to occur. Once such drying is complete,the outer surface layer is subjected to an elevated temperature forcuring and for evaporating therefrom the moisture preservation anddrying control agent to thereby produce a cured-ceramic coated substratewhose coating is void of large cracks and whose durability is enhanced.

[0010] Operationally, the application of the moisture preservation agentprevents formation of a weak, dry crust or skin during ambient drying ofthe coating. In particular, the agent combines with suspension water inthe outer layer of the coating to thereby displaces water at thesurface, limit dehydration, and reduce the gradient in water contentthrough the thickness of the coating. This reduced gradient evens outshrinkage throughout the thickness of the coating both duringambient-environment drying and early-stage elevated-temperature curing.During curing of the coating, and because of its outer surface-layermodification, the drying control agent acts to delay curing of the outersurface thereof to thereby ward off associated shrinkage and distributesdrying stresses. This delay slows onset of cracking, provides moreuniform curing throughout the thickness of the coating, and produceshealing of any cracks that do form by causing a flow of material fromthe outer layer into the cracks. Continued elevated temperature exposureduring the cure process evaporates the moisture preservation agent whichultimately results in allowing the cure of the outer layer. Theresulting coating has minimal curling and has narrow cracks at mostwithout any significant branching thereof parallel to the so-coatedsubstrate, thus producing a coating with quality and durability duringthermal reactivity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] An illustrative and presently preferred embodiment of theinvention is shown in the accompanying drawings in which:

[0012]FIG. 1 is a side elevation view in section of a non-cured silicatecement coating applied onto a substrate; and

[0013]FIG. 2 is a side elevation view in section of the silicate cementcoating applied onto the substrate as in FIG. 1 after the silicatecement coating is cured.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] Referring first to FIG. 1, a substrate 10 is shown with anon-cured silicate cement coating 12 a applied thereon. The coating 12 ais partially dried (about one hour in ambient conditions) to form amechanically stable outer surface layer 14 upon which a moisturepreservation and drying control agent 16 then is generally uniformlyapplied by brushing onto the outer surface layer 14. The agent 16 ishere shown prior to its penetration into the coating 12 a up to about0.010 inch and preferably is a mixture of glycerol and propylene glycol,most preferably in a 1:1 or 7:3 ratio by volume. Where a 1:1 ratio ofglycerol and propylene glycol is chosen, continued ambient drying ispermitted to occur for five to six additional hours, whereupon curing atan elevated temperature as dictated by the requirements of theparticular ceramic material of the coating 12 b proceeds to completionas depicted in FIG. 2. Any cracks 18 formed within the coating 12 b areone-half or less the width of an identical procedure but withoutapplication of the drying control agent 16. Further, no curling developsand no cracks parallel to the substrate 10 occur. Where a 7:3 ratio ofglycerol and propylene glycol is chosen, similar results are obtainedwhen a curing step having longer cure time or higher cure temperature isemployed for elimination of the increased amount of glycerol. In thismanner, a durable, ceramic-coated substrate-product is produced forlongevity and dedicated use in high thermally-variable environmentswhich are especially exemplified in aircraft engine components.

[0015] While an illustrative and presently preferred embodiment of theinvention has been described in detail herein, it is to be understoodthat the inventive concepts may be otherwise variously embodied andemployed and that the appended claims are intended to be construed toinclude such variations except insofar as limited by prior art.

What is claimed is:
 1. A method of controlling drying stress duringshrinkage of a ceramic coating coated onto a substrate, the methodcomprising the sequential steps of: a) applying a non-cured ceramiccoating onto the substrate and permitting the coating to dry until amechanically stable outer surface layer of the coating is formed; b)applying a drying control agent generally uniformly onto the outersurface layer of the coating in a quantity sufficient to penetrate intosaid surface layer for inhibiting formation of a dry outer skin duringdrying and thereafter continuing said drying; and c) subjecting theouter surface layer to an elevated temperature for curing said surfacelayer and evaporating therefrom the drying control agent.
 2. A method ofcontrolling shrinkage as claimed in claim 1 wherein the quantity ofdrying control agent applied onto the outer surface layer is limited topermit penetration to 0.010 inch.
 3. A method of controlling shrinkageas claimed in claim 1 wherein the drying control agent is a mixture ofglycerol and propylene glycol.
 4. A method of controlling shrinkage asclaimed in claim 3 wherein the mixture by volume of glycerol andpropylene glycol is 1:1.
 5. A method of controlling shrinkage as claimedin claim 3 wherein the mixture by volume of glycerol and propyleneglycol is 7:3.